Scandium halide discharge lamp



Nov. 7,- 1967 RELATIVE SPECTPHLJNTENS/TY ,Q

A. BAUER 3,351,798 SCANDIUM HALIDE DISCHARGE LAMP Filed Aug. 15, 1963700 WHVELNGTH (M/LL/M/CEONS) lnvervtor:

His A t torneg AvnQL'd B uev United States Patent 3,351,798 SCANDIUMHALIDE DISCHARGE LAMP Arnold Bauer, Augsburg, Germany, assignor toPatent- Treuhand-Gesellschaft fur Elektrische Glulrlampeu mbH Filed Aug.15, 1963, Ser. No. 302,376

Claims priority, application Germany, Aug. 22, 1962,

6 Claims. (Cl. 313-225) The present invention relates to high pressuredischarge lamps having solid hot electrodes and containing a fillingwhich includes a compound of a metal excited in the high pressuredischarge.

Amongst high-pressure lamps, the one most generally used is the highpressure mercury vapor lamp. It comprises an envelope consistinggenerally of quartz glass which contains a starting gas and a smallquantity of mercury which is completely vaporized in operation of thelamp. The arc maintained at a vapor pressure of 1 to 25 atmospheresemits, besides a relatively weak continuum, primarily a line spectrumconsisting of a few very strong lines. As a result, the light output ofthese lamps differs very much from natural light and also from the lightemitted by an incandescent solid body, and color rendition is quiteunsatisfactory. In order to improve the color rendition, it is alreadywell known to add to the mercury other metals, as zinc or cadmium, forthe purpose of increasing the percentage of red light emitted from thedischarge. Such additions, however, cause a substantial decrease inlight output which offsets the advantage gained in respect of colorrendition.

In mercury vapor lamps of former construction provided with liquidmercury electrodes, the addition of salts, e.g. of halides, by means ofwhich the mercury arc should get a definite coloration, was also known.

It has recently been proposed to provide in high-pressure mercury vapordischarge lamps with solid electrodes, in addition to the mercuryfilling, halide compounds of other metals which are excited to emissionsimultaneously with the mercury. Mercury vapor discharge lampsmanufactured according to this proposal contain in most cases anaddition of sodium iodide, potassium iodide, lithium iodide or thalliumiodide. They show a satisfactory filling up of the deficiency in red ofthe mercury are without the decrease in light output occurring in priormercury vapor lamps upon addition of zinc or cadmium. Since however noneof the excited metals emits many more lines than mercury, the visiblespectrum consists notwithstanding of a few lines only and is notequivalent to daylight in respect of color quality.

There may be obtained an excellent color rendition with a high-pressuredischarge lamp in which rare earths are excited because their spectrahave a close sequence of lines. However rare earths have thedisadvantage of a very low vapor pressure, so that in order to obtainhigh light output the discharge envelope must have temperatures of more1000 C. Thus by way of example, lanthanum iodide has a boiling point of1400 C.

Within the scope of experiments with rare earth additions, it has beensurprising to find that scandium compounds hold an exceptional positionas against all other metals or metal compounds hitherto used for thefilling.

If scandium is excited to radiant emission with definite intensity thenit emits in the visible range just as close a light spectrum as the rareearths but Without any large energy losses in the ultraviolet andinfrared ranges. Besides, the vapor pressure of some scandium compounds,especially that of scandium iodide ScI sufiices for a high pressure are.Thus, high-pressure lamps containing Sccompounds have two decisivecharacteristic features which have hitherto, not been foundsimultaneously in any highpressure lamp:

(1) The addition renders a light color which is extremely similar tonatural daylight, in the same fashion as the light of a high-pressurexenon discharge lamp. This may be seen from FIG. 1 which shows thespectral energy distribution of a high-pressure lamp with an addition ofS013.

(2) While additions of metal halides proposed hitherto have no badinfluence on light output but do not greatly increase it, now howeverthe efiiciency is raised to 105 lumens per watt (lm./w.) by addition ofa Sc-compound.

For lamps with high output concentration and resulting high walltemperature, the envelope of which consists of quartz glass orfine-crystalline, transparent aluminum oxide and in which the scandiumhalide vapor pressure exceeds 1 atm., a filling of $01,, by way ofexample, is sufiicient. For the purpose of better starting, anadditional rare gas filling at 10 to mm. of mercury cold pressure isdesirable.

In case of wall temperatures below 900 C. as are usual for instance inordinary high-pressure mercury lamps, the vapor pressure of ScI is below1 atm. Thus for instance at 800 C., it amounts to 0.1 atm. Therefore, itis desirable to fill-in additionally with a heavier buffer gas at higherpressure in order to reduce losses due to heat conduction and diffusion.The exciting voltages of Se are low, generally below 4 volts. Therefore,a suitable buffer gas is a heavy gas having exciting voltages lyingabove this limit. The heavier rare gases for instance meet thisrequirement. Generally, an addition of mercury vaporizing completely inoperation will be most suitable which ensures a partial pressure of morethan 1 atm., e.g. of 5 to 25 atm. The average exciting voltage ofmercury lies at about 7.8 volts and that of scandium below 4 volts, andan equal number of atoms of both elements are excited with a partialpressure ratio p /p -10 Provided the above-1nentioned ratio p /p =25/0.1=250 or less, scandium radiation predominates.

In an electric high-pressure vapor discharge lamp with solid hotelectrodes comprising an envelope consisting of refractory orhigh-melting, light transmissive material and containing a fillingincluding a metallic compound, according to the present inventionscandium is chosen as the added compound to be excited. The scandiumcompound may consist of Sel ScBr ScCl or of yet other scandium compoundswith sufficient vapor pressure such as Sc S In a lamp with tungstenelectrodes, the preferred compound is ScI because the iodine cycle thenprevents vaporized tungsten from depositing on the bulb wall. Theloading of the electrodes is desirably high enough that the tips orenvironments of the arc appendage thereon are molten in operation of thelamp.

The present invention provides a high-pressure lamp with excellent lightoutput exceeding even that of a highpressure mercury arc, and which hasa very good color rendition comparable with that of a highpressure xenonarc. Because of the relatively high vapor pressure of scandium halides,the lamp may be rated for an operating temperature as high as is usualfor the well-known high-pressure mercury lamps. These lamps may,therefore, in advantageous manner he produced according to welldeveloped methods of manufacture.

Beside scandium halide and, if desired, a quantity of mercury which iscompletely vaporized in operation, the lamp contains for the purpose ofbetter starting a rare gas filling at some 100 mm. of mercury coldpressure. A small excess of halogen over the stoichiometric ratiocorresponding to the scandium halide is desirable and may be introducedinto the lamp for instance in the form of mercury halide.

Owing to the fact that a discharge is scandium contracts more than adischarge in mercury, the manufacture of a wall-stabilized lamp notartificially cooled is quite difiicult. A lamp according to theinvention of high wall loading which is cooled artificially can be madeas a capillary lamp. The diameter of the discharge space amounts to afew millimeters at most; the length of the arc, up to some centimeters.

For cases in which artificial cooling is not desired,electrode-stabilized lamps have been made. Wall loading in these amountsof 20 to 60 w./cm. preferably 40 w./cm. The interelectrode distance orgap for projection lamps may amount to some millimeters. The lamp bulbis spherical or oval like that of well known extra high-pressure mercurylamps. If the operating mercury vapor pressure therein is higher andamounts for instance to 25 atmospheres, then with an electrode distanceof 6 mm., an operating voltage of 90 to 100 volts results. In lamps nothaving such high efiiciency, the operating pressure of the mercury vaporneed not be so high. Thus an operating voltage of 150 to 25 volts isobtained with a vapor pressure of -10 atmospheres and an interelectrodedistance of 1 to 2 cm.

FIG. 1 shows the spectral energy distribution of a high-pressure lampwith an addition of S01 FIG. 2 shows by way of example a lamp embodyingthe invention.

As illustrated in FIG. 2, the discharge envelope 1 consists of quartzglass and encloses a volume of 27 cm. The inner diameter of the tubularenvelope which is closed spherically at both ends amounts to about 30mm. The pin electrodes 2 and 3 consist of refractory metal, preferablyof tungsten wire 1.2 mm. in diameter and have the usual foil inleads 4and 5. The electrodes may be provided with tightly fitting quartzsleeves 6 and 7 passing integrally over into the quartz envelope 1 inorder to suppress the iodine cycle at cooler places on the electrodeswhere tungsten sputtering may occur. Any electrode activation beyond theusual thorium addition should be avoided. The rare gas filling in orderto facilitate starting amounts to mm. Hg of argon. The mercury fillingvaporizing completely in operation and amounting to 304 milligramsresults in a vapor pressure of about 10 atms. A filling of mg. ScI doesnot vaporize completely in operation. An addition of 3 mg. HgI bringsabout better behavior during life of the lamp.

The electrical and lighting data of the lamp are:

Wattage watts 2430 Current intensity amperes 13.3 Voltage volts 240Electrode distance cm 1.5 Wall load w./cm. Luminous flux lumen 254,000Efliciency lm./W. 105

An increase in output may easily be obtained with this lamp type byenlarging the lamp envelope and/or by means of forced cooling. Wallloading may be increased in simple manner up to 50% to 70% by puttingthe lamp into a glass tube 8 arranged vertically and open at both ends.The tube may be elongated upwardly for instance a length of 40 cm. andproduces a cooling air fiow by chimney effect. With this arrangement,there may for instance be converted an electric energy of 10 kw. in aquartz burner having a length of 9 cm. and a diameter of 5.8 cm.

Beside the type with short are distance there was developed another typewith greater arc distance which is operated with proportionately highvoltage of for instance 400 to 600 volts. It has been found that suchlamps require magnetic field stabilization.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A high pressure electric discharge lamp comprising an envelope ofrefractory light-transmitting material having a pair of slender solidtungsten electrodes sealed therein and containing a filling of mercuryproviding a vapor pressure in excess of 1 atmosphere when fullyvaporized in normal operation, a quantity of scandium iodide in excessof that vaporizing during operation, and a rare gas at a cold fillingpressure between 10 and millimeters of mercury.

2. A high pressure electric discharge lamp as defined in claim 1 whereinthe partial pressure of mercury is in the range of 5 to 25 atmospheresand the ratio of partial pressure of mercury to partial pressure ofscandium during operation does not exceed 250 in order to have scandiumradiation predominating.

3. A lamp as defined in claim 1 wherein the filling contains additionaliodine over the stoichiometric proportion corresponding to the scandiumiodide present.

4. A high pressure electric discharge device comprising an arc tubehaving electrodes sealed at either end thereof and a vaporizable fill ofiodine, mercury and atoms of scandium; said iodine and mercuryrespectively being present in an atomic ratio of about 0.14 and saidmercury being present in sufiicient quantities to be completelyvaporized at normal operating temperatures of said are tube and to forma restricted arc therein, the atoms of said scandium being present insufficient quantities to produce white light.

5. A high pressure electric discharge lamp comprising an envelope ofrefractory light-transmitting material having a pair of tungstenelectrodes sealed therein and containing a filling of mercury providinga vapor pressure in excess of one atmosphere when fully vaporized innormal operation, a quantity of a scandium compound in excess of thatvaporizing during operation, and a rare gas at a cold filling pressurebetween 10 and 100 millimeters of mercury.

6. A high pressure electric discharge lamp comprising an envelope ofrefractory light-transmitting material having a pair of tungstenelectrodes sealed therein and containing a filling of mercury providinga vapor pressure in excess of 1 atmosphere when fully vaporized innormal operation, a quantity of a scandium halide in excess of thatvaporizing during operation, and a rare gas at a cold filling pressurebetween 10 and 100 millimeters of mercury.

References Cited UNITED STATES PATENTS 2,673,944 3/1954 Francis 3l32283,234,421 2/1966 Reiling 313229 X FOREIGN PATENTS 900,200 7/1962 GreatBritain.

OTHER REFERENCES PublicationReference Book of Inorganic Chemistry,Latimer and Hildebrand, published by Macmillan Co., copyright ThirdEdition, 1051, page 453 relied upon.

S. D. SCHLOSSER, Primary Examiner.

GEORGE N. WESTBY, Examiner.

1. A HIGH PRESSURE ELECTRIC DISCHARGE LAMP COMPRISING AN ENVELOPE OFREFRACTORY LIGHT-TRANSMITTING MATERIAL HAVING A PAIR OF SLENDER SOLIDTUNGSTEN ELECTRODES SEALED THEREIN AND CONTAINING A FILLING OF MERCURYPROVIDING A VAPOR PRESSURE IN EXCESS OF 1 ATMOSPHERE WHEN FULLYVAPORIZED IN NORMAL OPERATION, A QUANTITY OF SCANDIUM